1. Field of Application
The present invention relates to a method of inspecting insulators to detect defects. The term “insulator” as used herein, unless otherwise indicated, signifies a solid electrically insulating member, such as the insulator portion of a spark plug of an internal combustion engine. It is assumed in the following that each insulator which is to be inspected has an aperture formed therein, such as the central aperture which extends axially through a spark plug insulator.
2. Description of Prior Art
A prior art method of insulator defect detection is known whereby a first electrode which is needle-shaped is inserted into a central aperture of an insulator such as a spark plug insulators, and a second electrode which is also needle-shaped is disposed at a distance from the second electrode such that a spark discharge can occur between the first and second electrodes. A high voltage is then applied between the first and second electrodes while the insulator is being rotated, so that repetitive spark discharges occur between the first and second electrodes. When this occurs, a judgement is made as to whether or not the insulator is defective, based upon whether or not the spark discharges take place along a predetermined path.
That is to say, if there is no defect in the insulator, then the spark discharges between the tips of the first and second electrodes will occur normally, whereas if the insulator is defective, then the current flow of the discharge will pass through the defective portion of the insulator. Thus, it is possible to determine whether or not the insulator is defective, by observing the path taken by the spark discharges.
With such a method it is essential to ensure that the spark discharges will occur along predetermined paths, between the tips of the first and second electrodes. However when a high value of potential difference is established between the first and second electrodes for producing such spark discharges, if the potential difference is excessively high then the discharges may occur along other paths, i.e., passing through metal portions of the inspection apparatus. For that reason, there are limitations on the values of high voltage which can be used in such an inspection method. Hence, it is difficult to ensure that insulators can be tested for providing a very high degree of electrical insulation.
Furthermore with such a insulator defect inspection method, since each of the first and second electrodes is formed with a needle configuration in order to generate spark discharges, it is necessary to rotate the insulator with respect to the second (i.e., external) electrode in order to inspect the entire periphery of the insulator. Hence it is necessary to provide a motor-driven apparatus for rotating each insulator that is to be inspected, while spark discharges repetitively occur around the periphery of the insulator. That is to say even if a large batch of insulators are to be inspected, it is necessary to use such an apparatus (or plurality of apparatuses) to individually rotate each of the insulators. This is a basic disadvantage of such a prior art insulator defect inspection method.
Furthermore, due to the fact that the tip of the first electrode is located at one specific position within the aperture in the insulator that is being inspected, and any by-passing of the spark discharges (due to current flowing through a defect region in the insulator) will take place along a path between the respective tips of the first and second electrodes, the range of positions in the insulator which are effectively inspected by such a method is limited.